Alternating-flow heat exchangers are heat exchangers in which gas or fluid may flow in at least two flow paths through the heat exchanger. Such heat exchangers have a variety of uses, such as, for example, as catalytic converters, furnace heat recuperators, turbine engine recuperators, and in process fluid heating or cooling. The same structure may also be used for cross-flow filters or bleed-through reactors.
Heat exchangers are typically formed by first extruding a honeycomb like body of ceramic material from a die orifice. The extrusion results in a block of ceramic material having flow channels or cells which are typically of square or other rectangular cross-section, arranged parallel and adjacent to one another along the axis of extrusion. To form alternating-flow heat exchangers, portions of the sides of the extruded block are commonly cut away to convert the ceramic block having only straight-through passages into a ceramic block alternating between rows of straight-through flow (primary flow), and rows of Z-flow, L-flow, U-flow or other similar alternate or cross directional flow (secondary flow) through the ceramic block.
The secondary-flow (Z-flow, L-flow, etc.) channels are typically made by sawing into the sides of some of the channels in the ceramic block and afterwards sealing the ends of these channels, thereby forming the secondary-flow channels. Examples of methods which utilize such sawing techniques to form Z-flow alternating-flow heat exchangers are disclosed in U.S. Pat. No. 4,271,110 to Minjolle, and U.S. Pat. No. 4,421,702 to Oda et al. U.S. Pat. No. 4,298,059 to Krauth et al. discloses a similar sawing method, wherein diamond cutting wheels are used to saw slots into an extruded body, after which time the ends of the slots are plugged to form an L-flow alternating-flow heat exchanger in which both flow directions through the heat exchanger follow an L-shaped path.
Such sawing techniques make it extremely difficult to produce high-quality alternating-flow devices, especially those having thin flow channels, and/or which alternate direction in every adjacent row (i.e. single alternating-flow channels or cells). This is because, to produce the secondary-flow channels in such devices, cell walls in the secondary-flow channels must be removed without sawing into adjacent walls. Such precision sawing is difficult in part because the prior art sawing techniques utilize large planar sawing devices, such as diamond cutting wheels. Such large planar cutting wheels must align with the channel to be removed, so that every point on the cutting wheel is at all times accurately aligned with each cell area to be removed. Otherwise, the cutting wheel will remove cell walls from adjacent cells, causing leakage problems. Accurate cutting is made more difficult by the fact that such planar cutting devices are susceptible to chattering, or vibration, during the cutting operation. Such chattering of the cutting wheel can, for example, cause the outer edges of the wheel to cut differently than the center area of the wheel.
Perhaps a bigger disadvantage occurs because, in the past, diamond wheel and other sawing techniques were used to remove entire channel sections from the ends of the extruded body. Consequently, once the cutting operation was complete, the end of the extruded body alternated between the still existing structure of the straight-through flow layers, and open gaps produced by the cutting wheels. To complete the formation of the secondary-flow channels, these relatively large open ends or gaps had to be plugged. Because of the relatively large section to be plugged, bars of material were typically employed which were wide enough to fit into these gaps, yet not so wide that they would force adjacent cells further apart. These plugging bars therefore had to be formed to a very exact tolerance, so that they fit accurately and consistently into each gap. This consistency of fit was sometimes made difficult by the fact that the sawing techniques, as explained above, do not always cut consistent width channels. Because of the large spaces which must be plugged, as well as the problem associated with plugging them, shrinkage voids, slumped plugging paste, and drying cracks in the plugging paste are not uncommon problems with these structures. Also, the larger the area to fill or plug, the more critical it will be to match thermal expansion coefficients between the filling and body materials. These differences in thermal expansion can result in cracking which results in leakage.
Also, because of the relatively large gaps left in the extruded structure between the straight-through flow channels, these structures are very weak prior to plugging. Consequently, care must be taken during manufacture to prevent bending of the ends of the straight-through channels prior to plugging. This is a problem regardless of whether sawing takes place prior to or after firing, but it is a particular problem when sawing takes place prior to firing, given the already relatively weak nature of the green ceramic ware.
The present invention is directed to a method for forming which avoids the deficiencies of prior art methods for forming alternating-flow heat exchangers.